The unexpected kinematics of multiple populations in NGC6362: do binaries play a role?

TL;DR

This study reveals that binary star populations significantly influence the observed kinematic differences between multiple stellar populations in NGC6362, challenging previous explanations based solely on spatial distribution and anisotropy.

Contribution

It demonstrates that binary star fractions can account for the kinematic differences between populations, supported by multi-epoch RV data and N-body simulations.

Findings

FG stars have a higher binary fraction (~14%) than SG stars (<1%).

Binary stars inflate the velocity dispersion of FG stars.

Kinematic differences are explained by binary fractions, not spatial distribution.

Abstract

We present a detailed analysis of the kinematic properties of the multiple populations (MPs) in the low-mass Galactic globular cluster NGC6362 based on a sample of about 500 member stars for which radial velocities (RVs), Fe and Na abundances have been homogeneously derived. At distances from the cluster center larger than about 0.5 $r_h$, we find that first (FG - Na-poor) and second generation (SG - Na-rich) stars show hints of different line-of-sight velocity dispersion profiles, with FG stars being dynamically hotter. This is the first time that differences in the velocity dispersion of MPs are detected by using only RVs. While kinematic differences between MPs in globular clusters are usually described in terms of anisotropy differences driven by the different radial distributions, this explanation seems hardly viable for NGC6362, where SG and FG stars are spatially mixed. We demonstrate that the observed difference in the velocity dispersion profiles can be accounted for by the effect of binary stars. In fact, thanks to our multi-epoch RV measurements we find that the binary fraction is significantly larger in the FG sample (f~14%) than in SG population (f<1%), and we show that such a difference can inflate the velocity dispersion of FG with respect to SG by the observed amount in the relevant radial range. Our results nicely match the predictions of state-of-the art $N$-body simulations of the co-evolution of MPs in globular clusters that include the effect of binaries.